A humerus-scapular joint (referred to herein as a shoulder joint) prosthesis comprises a humeral component having a stem part which can be fitted into a reamed cavity within the medullary canal of the humerus, and a glenoid component for attachment to the glenoid. The humeral component and the glenoid component comprise corresponding bearing surfaces which articulate together as the joint moves. In a natural shoulder joint the humeral component comprises a convex head, which articulates against a concave bearing surface on the glenoid. This structure is reproduced in an “anatomic” shoulder joint prosthesis, in which the humeral component includes a stem part and a head part with a convex bearing surface and the glenoid component provides a concave bearing surface. The stem part is implanted within the humerus. The head part is fitted to the stem part, or is formed integrally with the stem part, so that it sits above a resection surface of the humerus. Anatomic prostheses are suitable for implantation in patients where joint tissue has degraded (for example, due to arthritis).
The structure of the anatomic joint is reversed in a “reverse” shoulder joint prosthesis. The glenoid component includes a convex head, and the humeral component has a concave recess in the epiphysis, in which the head on the glenoid component can be received and articulate. The humeral component of a reverse joint prosthesis, including the epiphysis part which provides the bearing surface, may be implanted almost entirely within the humerus.
The biomechanical properties of the patient's joint are altered when a reverse shoulder joint prosthesis is implanted because the center of rotation of the joint is shifted medially. A reverse shoulder joint prosthesis is suitable for implantation in a patient with damaged cuff muscle tissue. The shift of the center of rotation allows manipulation of the arm using the deltoid muscle because of the increased mechanical advantage.
A reverse shoulder prosthesis is described in WO-2007/039820 (DePuy (Ireland) Ltd). Such a joint prosthesis is available commercially and sold by DePuy Products Inc. under the trade name Delta Xtend.
After implantation of a reverse shoulder prosthesis it may be necessary to perform a revision procedure in order to revert the shoulder joint to an anatomic configuration. Alternatively, it may be necessary to revert to an anatomic configuration part way through the implantation of a reverse shoulder prosthesis in the event that it becomes apparent that there is insufficient bone stock to support the reverse shoulder prosthesis. Embodiments of the present invention relate to surgical instruments, and in particular a reaming guide, that may be used during such a revision procedure.
It is an objection of embodiments of the prior art to obviate or mitigate one or more of the problems of the prior art, whether identified herein or elsewhere.